[STORY] Read-Only Enforcement Middleware

**Estimate**: 3 pts

## Acceptance Criteria
- Capability negotiation on handshake
- Block all write operations
- Protocol-level enforcement

No files or tests are named. Start by locating the protocol handshake and middleware paths for write operations, then read how capabilities are represented. Done means read-only capability negotiation exists on handshake and protocol-level write operations are blocked, but confirm scope with the assignee before starting.

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